Clinical trials of agents to preserve beta cell function in new onset Type 1 diabetes will use C-peptide as an outcome measure. The compatibility of C-peptide results among different methods and laboratories has become a problem for many international studies involving C-peptide measurement. In 2001, the ADA convened an international C peptide workshop at which participants agreed on the need for C-peptide standardization across laboratories to facilitate the conduct of clinical trials. In response to this request for standardization, a C-peptide standardization committee was established and international C-peptide comparison studies were conducted. These studies have shown that although calibration with pure C-peptide standards (WHO IRR 84/510) did not successfully improve comparability, patient samples could successfully be used to calibrate assays and reduce variability. In addition, a reference method for C-peptide was identified and subsequent studies have shown that normalization to this LC/MS reference method can be used to improve comparability of results (Clin Chern 2007;53 :784-7/Clin Chern. 2008;54: 1023- 1026).